Foils and sheets which have a thickness below 200 mm, when drawn have an elongation which is very low and they are also apt to wrinkle. Therefore the drawing of these foils and sheets is very difficult. However, in the field of packaging of foods, medicines, electric parts and so on, the use of such foils and thin sheets is desirable from the point of economy, light weight, disposability and so on.
They are not used widely because of their poor drawability, however, i.e. because they are apt to wrinkle and can not be drawn with large drawing ratio.
There has previously been disclosed in Japanese patent No. Sho 56-50645 a punch having a rigid core and an elastic body. While that punch is an improvement over a conventional punch having a rigid body, the drawing ratio obtainable with that punch is still relatively low.
The concepts of drawing, including drawing ratio and wrinkle, will be described with reference to FIG. 1 so as to be able to give a clear explanation of the present invention.
The static drawing ratio is the ratio obtained by dividing the initial diameter of the blank by the punch diameter Dp, namely, initial blank diameter/Dp.
On the other hand, from the dynamic point of view, the drawing ratio is the ratio of the strength of the material being drawn to the force required for drawing.
The force required for drawing mainly comprises three kinds of forces.
One is the force required for shrink deformation of the material being drawn on the die face 3. This force is porportional to the blank diameter.
The second is the friction force between the die 3 face and the material being drawn and between the material being drawn and the face of the blank holder 2. The last one is the force for bend working of the material being drawn at the die shoulder. The total of these three forces increases gradually in the early stage of drawing due to the effect of the work hardening of the material being drawn, reaches a maximum about half way through drawing process, and then gradually decreases.
On the other hand, the strength of the material relevant to the drawing ratio is the strength determined by the product of the circumferential sectional area of the material being drawn and the unit tensile strength, namely thickness x circumferential length x unit tensile strength, at a fixed point in the axial direction.
The wall breakage of the foil generally occurs at the point where the strength is minimum. The practical drawing ratio, that is to say the limiting drawing ratio, depends on both the minimum strength of the product and the maximum total drawing force. The minimum thickness of the material being drawn is near the bottom of the material being drawn, namely, near the container bottom in the axial direction of the container, i.e. the drawing direction.
The thickness increases gradually toward the open end of the container.
The material strength varies in the same manner as the change of thickness.
As stated, the maximum drawing force appears at about half way through the drawing process, the minimum point of material strength exists near the container bottom, and the wall breakage generally happens at a point at which the drawing is rather far along and at the portion near the periphery of the container bottom.
There are two types of wrinkles.
One is generated at the flange portion (hereinafter called flange-wrinkles) and the other is generated in the wall portion B (hereinafter called wall-wrinkles).
From the dynamic point of view, it may be said that a wrinkle is a phenomenon which occurs when the holding force perpendicular to the material surface being drawn can not restrain the buckling of the material caused by circumferential compression stress.
Therefore, flange-wrinkles can be restrained by increasing the holding force (hereinafter called blank holder force) between blank holder 2 and die 3.
However, the friction between the blank holder, the die and the material being drawn also increases in accordance with the increase of the blank holder force and as a result wall breakage occurs more readily.
Therefore, it would seem that the initial blank diameter (and hence the static drawing ratio) should be reduced by the equivalent of the blank holder force increment for restraining of wrinkles. However, it is common knowledge that the improvement of wrinkles and drawing ratio can not be simultaneously overcome in this way.
On the other hand, the generation and the degree of wall-wrinkles depend on the clearance between punch 1 and die 3. When the clearance is greater, the wall-wrinkles are heavier.